1. Technical Field
The embodiments herein generally relate to filtration media, and more particularly to metal-organic frameworks (MOFs), and the process of removing toxic chemicals, such as ammonia, using plasma-enhanced fluorocarbon MOFs.
2. Description of the Related Art
Sorbents are currently employed in numerous fields, including gas (e.g. hydrogen, carbon dioxide, methane)/liquid storage and separations, as well as filtration. Current military and first responder respirator and building filters typically employ activated, impregnated carbons for the removal of toxic chemicals, including chemical warfare agents (e.g. GB, GD, VX, HD, etc) and toxic industrial chemicals (e.g. ammonia, chlorine, hydrogen chloride, sulfur dioxide). In filters, the adsorbent is housed in a structure such that a toxic gas stream passes through a packed bed, monolith, or volume such that the toxic gas contacts the adsorption media and is removed by physical adsorption and/or chemical reaction. Although activated carbon is an excellent adsorbent for the removal of toxic chemicals, it can fall short in areas involving the removal of a full spectrum of toxic chemicals. Impregnants may initially be active towards many classes of toxic chemicals; however, when exposed to variant temperature and humidity, impregnants may interact with one another, causing aging and degradation of activity.
MOFs are a relatively new class of porous materials comprised of metal centers (or clusters) and organic linkers. These can be visualized as a series of joints (metal clusters) and struts (organic linkers) that form an extended, porous network. A wide variety of MOFs are available by interchanging the metal cluster and organic linkers. Moreover. MOFs are tailorable such that they can be designed from the “bottom-up” on a molecular level. MOFs can be made into 12-, and 3-dimensional structures. Although MOFs are incredibly attractive for a variety of applications, many, most notably those containing carboxylate ligands, are air sensitive. Specifically, many degrade in the presence of nucleophiles such as water (liquid or humid air) due to hydrolysis or ammonia due to aminolysis of the MOF structure as described by Peterson, G. W., et al., “Ammonia Vapor Removal by Cu(3)(BTC)(2) and Its Characterization by MAS NMR,” J. Phys. Chem. C, vol. 113, pp. 13906-13917 (2009), the complete disclosure of which in its entirety, is herein incorporated by reference. Many efforts have focused on developing MOFs that are air-stable and water-stable. Most of these efforts focus on changing the type of metal or type of linker such that stronger bonds result or moisture is repelled. Examples of this include creating MOFs comprised of zirconium and titanium metal centers, which result in metal clusters that are more stable, as described by Cavka, J., et al., “A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability,” J. Am. Chem. Soc., col 130, pp. 13850-13851 (2008), the complete disclosure of which in its entirety, is herein incorporated by reference.
Linkers have also been changed by using cyclic organics containing nitrogen instead of carboxylic groups, and hanging hydrophobic functional groups from the linker to repel water. Generally, in all cases, these modifications result in an inherent change to the MOF structure, porosity, and other physical characteristics. Therefore, it is desirable to develop a technique that does not change the structure of the MOF while retaining the inherent characteristics of the MOF.
Additionally, the process of treating surfaces and microporous materials such as activated carbon and silica using plasma techniques have previously been described in U.S. Patent Application Publication No. 2010/0024643 published to Robert Harold Bradley on Feb. 4, 2010, the complete disclosure of which, in its entirety, is herein incorporated by reference. The disclosed process is aimed at changing the diffusion and wetting properties of the materials. Additionally, the publication describes increasing the hydrophobicity of materials to reduce moisture uptake.